My laptop was sitting idle with the fan at full tilt. Nothing was running that I knew of. The culprit turned out to be ten orphaned busy-loops left behind by a Claude Code session two days earlier.
Just fix it for me
If your fan is roaring right now and you would rather not read the rest, paste this into a Claude Code session:
My machine seems idle but the fan is at full speed. Check the load average against my core count, then list the top CPU consumers with their PPID, elapsed time, and full argument list. I am looking for orphaned processes: anything reparented to PID 1 that has been burning CPU for hours or days, especially shells and interpreters where the process name alone tells you nothing. Show me what you find and what each one actually is before killing anything, and let me confirm the list first.
The "show me before killing" part is the important bit, and it is why the prompt is written that way rather than as "find and kill whatever is using CPU." A long-running process pinned to PID 1 is a strong hint that something was abandoned, but it is not proof. Daemons legitimately live there, and your own nohup'd job or a detached build will look identical from a distance. Read the argument list before you agree to anything. In my case the arguments made it obvious in about two seconds.
Finding it
Start with the load average. This is a 10-core machine:
$ uptime
19:39 up 6 days, 6:14, 10 users, load averages: 122.91 167.84 162.08
Load 122 on 10 cores is not idle. Next, the top CPU consumers:
$ ps -Ao pcpu,pid,ppid,user,comm -r | head -12
%CPU PID PPID USER COMM
139.8 8320 1 user /Applications/Google Chrome.app/Contents/MacOS/Google Chrome
60.9 94281 1 user /bin/zsh
59.4 94279 1 user /bin/zsh
59.4 94288 1 user /bin/zsh
59.1 94287 1 user /bin/zsh
57.9 94285 1 user /bin/zsh
57.6 94284 1 user /bin/zsh
57.6 94286 1 user /bin/zsh
57.3 94283 1 user /bin/zsh
55.9 94282 1 user /bin/zsh
51.1 94280 1 user /bin/zsh
Ten zsh processes at roughly 60% each, and every one has PPID 1. That is the tell: their parent died and launchd adopted them. comm only gives you the binary name, so pull the full argument list to see what they actually are:
$ ps -o pid,lstart,etime,pcpu,args -p 94279,94280,94281
Note the comma-separated list. Passing -p alongside -A silently gets you every process on the box instead.
The arguments explained everything:
/bin/zsh -c source ~/.claude/shell-snapshots/snapshot-zsh-XXXX.sh 2>/dev/null || true && eval '
SP=/private/tmp/claude-501/<project>/<session-id>/scratchpad
# saturate all cores, then run the suite under contention
NCPU=$(sysctl -n hw.ncpu)
for i in $(seq 1 $NCPU); do (while :; do :; done) & done
LOADPIDS=$(jobs -p)
pnpm test:integration > "$SP/load.log" 2>&1
kill $LOADPIDS 2>/dev/null
...'
Elapsed time was 01-22:41:17. Just under two days of while :; do :; done on every core.
A previous session had deliberately pegged all ten cores to run an integration suite under CPU contention, then meant to clean up after itself. The test run itself was long gone, confirmed by the absence of any vitest or pnpm process:
$ ps -Ao pid,ppid,pcpu,etime,comm | grep -Ei "[n]ode|[v]itest|[p]npm"
58938 58933 0.0 00:59 .../bin/node
88656 88648 0.0 02-00:57:58 .../bin/node
Only the spinners were left. They accounted for about 700% of the 850% total CPU in use:
$ ps -Ao pid,ppid,pcpu,comm | awk 'NR>1 && $3>20 {sum+=$3; n++} END {print "procs >20% CPU:", n, " total %CPU:", sum}'
procs >20% CPU: 12 total %CPU: 850.3
Why the cleanup failed
Two things went wrong, and either alone would have been enough.
LOADPIDS=$(jobs -p) was the first. Job control is off in a non-interactive shell, so jobs -p returned nothing and the cleanup kill had no arguments to act on. Collect $! after each background spawn instead:
LOADPIDS=""
for i in $(seq 1 $NCPU); do
(while :; do :; done) &
LOADPIDS="$LOADPIDS $!"
done
The second is that the parent shell died before reaching the kill line at all. A cleanup step on the happy path is not cleanup. Use a trap so it fires even on interrupt:
trap 'kill $LOADPIDS 2>/dev/null' EXIT INT TERM
Fixing it
Plain kill was enough. No -9 needed:
$ kill 94279 94280 94281 94282 94283 94284 94285 94286 94287 94288
$ ps -o pid= -p 94279,94280,94281,94282,94283,94284,94285,94286,94287,94288 | wc -l
0
Afterwards, Chrome was back on top where it belongs and nothing else broke 30%:
$ ps -Ao pcpu,pid,comm -r | head -4
%CPU PID COMM
129.6 8320 /Applications/Google Chrome.app/Contents/MacOS/Google Chrome
43.2 58249 .../Google Chrome Helper (Renderer)
27.5 69725 .../UsageTrackingAgent
The load average still read 74 at that point, which is expected. It is a decaying rolling average and lags reality by design. The 1-minute figure had already dropped from 122 to 74 and kept falling; the 15-minute figure was still carrying the previous quarter-hour of saturation. Judge the fix by the process list, not the load average. The fan takes another minute or two beyond the CPU drop while the heat already in the chassis dissipates.
Checking your own machine
uptime # load average well above your core count?
sysctl -n hw.ncpu # what your core count actually is
ps -Ao pcpu,pid,ppid,user,comm -r | head -15
Look for processes eating CPU with PPID 1 and a long etime. Reparenting to launchd plus an elapsed time measured in days means nobody is supervising it and nobody is going to clean it up. Shells and interpreters are worth a closer look, since comm shows you /bin/zsh or node and tells you nothing about what is inside. Get the arguments:
ps -o pid,ppid,lstart,etime,pcpu,args -p <pid>
On macOS, Activity Monitor sorted by CPU shows the same thing, but the process name column has the same problem: ten identical zsh rows and no hint of what they are running.
Takeaway
Agents run real commands, including ones that deliberately consume resources, and a crashed or cancelled session does not necessarily take its children with it. If you let an agent spawn background work, it is worth knowing how to spot the leftovers. ps -Ao pcpu,pid,ppid,user,comm -r | head costs nothing and would have caught this two days earlier.